Due to the general inaccessibility of window blind head rails, and the complexity of modern blinds, it is necessary for window blinds to comprise a control unit, and many types are known in the art. The nature of the control unit will vary according to the type of blind with which it is being used, and the present invention relates in particular to improvements in a control unit suitable for use with a manually operated roller blind type head rail.
Head rails of this type may be used to hang any suitable blind where a control unit as described can be applied, but most commonly roller blinds.
A control unit for use with a roller blind type head rail will typically include a sprocket wheel in connection with a chain, a chain guard housing, and a sprocket support which facilitates controlled rotation of a splined bush. The sprocket wheel rotates in response to movement of the chain by the user, with the sprocket support providing a controlled and limited resistance to rotation. This in turn causes the splined bush to rotate. The control unit engages the roller blind tube, causing an attached blind to raise or lower as a result of the operation of the chain. In this way, movement of the blind from an open to a closed position is achieved without excessive stretching or discomfort to the individual.
This mechanism is exemplified by the roller blind control units sold by Louver-Lite Limited in five sizes corresponding to their System 32, 40, 40+, 45 or 45+ roller blind ranges.
This unit comprises a sprocket wheel and housing. The sprocket wheel interacts with a typical wrap spring having outwardly projecting end juts. The wrap spring is in turn connected to a sprocket support by a sprocket spring friction surface. The sprocket support includes two engaging pins projecting from one face of the control unit. These are designed to engage with a metal or plastic wall or ceiling-mounted bracket, thereby providing a means for hanging the blind in front of a window or other aperture.
In a variation on this design, as shown in UK design registration no 3006555, the sprocket support also features a radially extending projection which interacts with one of three distinct channels in the chain guard housing adapted to receive the projection. This allows the chain guard housing to be locked into one of three positions relative to the sprocket support when the blind unit is fitted. This stabilises the unit, with the chain guard housing aperture in an appropriate position in view of the location in which the blind is to be hung, whilst preventing independent movement of these two components. For example, if the blind is to be hung over a window with nothing positioned below the window to impede user access, the radially extending projection may be positioned to interact with the central channel. This allows the blind chain to hang along side and more or less parallel to the plane of the blind. However, if the blind is to be hung, for instance, at a kitchen window, behind a sink, the radially extending projection may be placed in one of the two side channels, causing the chain to project from the plane of the blind, and be reached more easily by the user.
In all of the above referred to embodiments, movement of the roller blind chain causes rotation of the sprocket wheel and releases the wrap spring clutch. The sprocket wheel component of this control unit also interacts with a splined bush which, as a result, rotates upon movement of the sprocket wheel. The external surface of the splined bush is connected with the roller blind tubing, thereby facilitating rotation and therefore raising or lowering of the blind as a result of movement of the chain.
The units are held together by a centre pin which extends substantially through the centre of the control unit. The head of the pin comprises a locking-lug of the centre-pin stop element typically in the form of co-operating flattened surfaces in what is otherwise a tubular interface between the centre pin and the sprocket support. The locking-lug of the centre-pin stop element engages the sprocket support and provides an additional point of connection with the mounting bracket. The hooked tip of the centre-pin stop element prevents the blind from being pulled out of the bracket if the chain is pulled at a non-orthogonal angle from the aperture of the chain guard housing. The splined bush snap fits over the centre-pin tips whereby the two locking lugs of the centre pin engage a centre-pin engagement surface of the splined bush.
However, there are features of the above design which could be improved. For instance, the chain guard housing of this design is known to suffer from damage when the chain is roughly handled or yanked by the user.
It is also desirable to provide a control unit that does not need to be dismantled in order to feed the chain into position with the sprocket wheel. Known control units are designed to be dismantled in order to complete this process. However, dismantling the unit is time consuming, and many blind assemblers force the chain to engage with the sprocket wheel without dismantling the unit, thereby risking damage to both the wheel and the housing.